1. Field of the Invention
The present invention relates to a measuring method and exposure apparatus. More particularly, the invention relates to exposure apparatuses (aligner, stepper, and others) required for the lithography process in fabricating semiconductor elements and the like, and to a method of checking the accuracy of the exposure apparatuses.
2. Related Background Art
In the lithography process in the fabrication of the semiconductor elements, circuit patterns and others formed on the original which is usually called photomask or reticle (hereinafter referred to as reticle collectively) are printed on the resist layer coated on a semiconductor wafer. Then, the required photoetching is performed with the resist pattern which is formed by the subsequent development process as its mask. In this process, particularly in using exposure apparatus therein, the reticle and wafer are set with a high-resolution projecting optical system therebetween, and subsequent to an accurate positioning of the reticle and wafer, the patterns on the reticle are exposed onto the wafer. In an exposure apparatus of this kind, there has been a trend that the integration of the circuit patterns to be printed on the wafer becomes increasingly higher and accompanying this tend, the width of the patterned lines has come into the region of sub-micron order.
Thus, in a stepper or other exposure apparatuses used for the lithography process which has become increasingly finer year after year, there has been provided an alignment system having a precision enabling an overall positioning on an order of several fractions to approximately 1/10 of the minimum line width in order to execute a highly precise positioning of the reticle and wafer. This alignment system comprises various alignment sensing systems for detecting the positioning (alignment) conditions between the reticle and wafer, a wafer staging system for stepping the wafer accurately, a reticle staging system for setting the reticle accurately, and others as its central components.
Also, there have been the methods given below for checking the accuracy of the exposure apparatus provided with the alignment sensing system, wafer staging system, and reticle staging system (such as the positioning accuracy of the wafer stage, the overlapping accuracy of the reticle and wafer, or the like).
(1) A test reticle provided with a special vernier pattern is prepared, and a double exposure is performed by displacing the wafer only by a predetermined amount so that the major scale and sub-scale of the vernier on the reticle are overlapped on the wafer. Then, the resist image of the vernier is observed by a microscope after its development.
(2) As disclosed in U.S. Pat. No. 4,803,524, two parallel linear patterns (diffraction grating type) are printed on the wafer at an interval, and subsequent to the development process, the interval between these two parallel linear patterns is self-measured by the alignment sensor in the exposure apparatus.
(3) As disclosed in U.S. Pat. No. 4,908,656, two sets of plural linear patterns set in parallel are doubly exposed so that these are overlapped at a predetermined angle to form a wedge-shaped overlapping portion. Then, the amount of the deviation is detected from the line width in such an overlapping portion.
In the conventional arts mentioned above, the following problems are encountered respectively.
(1) When the accuracy of the exposure apparatus is checked, the use of the vernier is employed for the method as if using slide calipers. As a result, the amount of the deviation and the measured value by the vernier become identical to each other 1:1, and depending on the observing method and others to examine the condition according to the state of the vernier pattern formation on the photosensitive board or optical microscope and other means, there is a possibly that a great measurement error is generated. Also, there are problems with the method of using the vernier patterns in that the calibrations should be read to check the misalignment in this method, thus making it difficult to automate the system. Not only is the processing speed slow, but the operator tends to get tired.
(2) In the method of measuring the interval between the two linear patterns, the measuring sensor should be provided with a high precision for detecting the resist images because the interval between the resist images of the linear patterns should be directly detected optically.
(3) In the method of measuring the line width of the wedge-shaped overlapping portion formed by the double exposure, the obtainable result is not accurate enough.
In the exposure apparatus of the kind, it is required to transfer a reticle pattern onto a wafer in a predetermined enlarged or reduced scale, and a mechanism for correcting imaging characteristics of a projection optical system, in particular, for correcting an error of a magnification or distortion is provided. In order to perform the correction, the error of the magnification amount or the distortion amount must be accurately measured first.
In a conventional exposure apparatus, in order to perform such a measurement, a specific pattern is formed on a reticle, and an optical member having a slit-like window is placed on an XYZ stage for three-dimensionally positioning the wafer. An image of the specific pattern on the reticle formed by the projection optical system and the slit-like window are converted into enlarged images having proper sizes, and the interval between the pattern image on the reticle and the slit-like window is measured based on the enlarged images. Then, the error of the magnification amount or the distortion amount of the projection optical system is obtained on the basis of a difference between the measured interval and a design value.
In the conventional measurement method, the XYZ stage and the measurement optical system are complicated. Since the interval between the image of the pattern on the reticle and the slit-like window on the XYZ stage is enlarged by the optical system, the measurement result often includes an error under the influence of aberrations of the optical system and a variation of characteristics of the optical system.